[RFC] Fix clang empty report showed when warnings present.

Problem  : Empty error is being showed even if there are warnings.
Cause    : scan-build --status-bugs is working, but checked status is
           that of the pipe, not scan-build's one.
Solution : Set option `pipefail` so that pipe status is that of the last
           failing command.
Instead we can check `${PIPESTATUS[0]}`. They're both bashisms so we might as well use the precise one.

```bash
scan-build \
       --status-bugs \
       --use-analyzer=$(which clang) \
       --html-title="Neovim Static Analysis Report" \
       -o build/clang-report \
       ${MAKE_CMD} \
       | tee ${BUILD_DIR}/scan-build.out
[ ${PIPESTATUS[0]} -eq 0 ] && scan_build_result=ok || scan_build_result=warnings
```